An electric vehicle and a plug-in hybrid vehicle include an inverter for driving a motor by a high voltage storage battery for driving power and a low voltage storage battery for operating an auxiliary machine such as a light and a radio of the vehicle. A power converter from a high voltage storage battery to a low voltage storage battery, specifically a DC-DC converter, is mounted on such a vehicle.
The DC-DC converter includes a high voltage side switching circuit for converting a DC high-voltage into an AC voltage, a transformer for converting an AC high-voltage into an AC low-voltage, and a low voltage side rectifier circuit for converting an AC low voltage into a DC voltage.
Generally, as a circuit configuration of a DC-DC converter, four MOSFETs are connected in an H bridge configuration as a high voltage side switching circuit, a smoothing capacitor is connected on an input side thereof, and a resonant chock coil is connected on an output line thereof. As a transformer, a center tap type transformer is used in which a middle point of a secondary side winding is pulled out to a winding outside. As a low voltage side rectifier circuit, a smoothing circuit including a chock coil and a capacitor is connected to a rectifier circuit using a diode or a MOSFET (refer to PTL 1 for example).
The above-described transformer includes a primary side winding (primary winding) connected to a high voltage side switching circuit, a secondary side winding (secondary winding) connected to a low voltage side rectifier circuit, and a magnetic body (core) for magnetically coupling the primary side and the secondary side and transmitting energy. Each winding is wound around a plastic bobbin for shape retaining and fixing support and attached to the core. Further, to enhance insulation of the primary winding and the secondary winding, an insulation tape and a spacer can be inserted between the primary winding and the secondary winding. As the winding, generally a copper wire such as an enamel wire is used. However, in a transformer using in a large output converter, the secondary winding is bonded on a copper plate such as a bus bar to increase a current capacity of the secondary side winding.
In the case where the above-described transformer is used in the large output converter, a temperature increase of the transformer becomes a problem. Since the primary winding and the secondary winding have a resistance component, a copper loss is caused by conduction, and heat is generated. When a DC-DC converter operates, an AC current corresponding to an operation frequency of the DC-DC converter flows in the primary winding and the secondary winding. Therefore, resistance components of the primary winding and the secondary winding increase by a skin effect in comparison with a DC resistance. Further, when the DC-DC converter operates, an AC magnetic flux is applied to a core of the transformer. Consequently, an iron loss is caused to the core, and heat is generated. As described above, since all of the primary winding, the secondary winding, and the core which are main components in the transformer are heated, the temperature increase becomes a problem.
To suppress the temperature increase in a transformer, the transformer is disposed via a grease and a heat conductive sheet on a case surface of a DC-DC converter including a cooling path or a radiation fin. Further, a resin material such as silicon rubber can be injected between windings and between a winding and a core to improve a heat radiation property. However, in a portion far from the case surface of the DC-DC converter, especially on an upper portion of the transformer, the transformer has a large heat resistance, and a temperature increase is not avoidable.
As a method for solving the temperature increase of a transformer, a two-transformer circuit configuration in which two transformers are used can be applied. When two transformers are used, a heat is generated dispersedly in the two transformers, and therefore, a temperature increase in each transformer is reduced. In the case where two transformers are used, usually, in each transformer, the primary winding is connected in series, and the secondary winding is connected in parallel (via a rectifier element) (refer to PTL 2 and 3 for example). By applying the above-described configuration, the number of turns required in the primary winding in each transformer is halved, and the height of a core of each transformer can be lowered by decreasing the number of winding turns. In the case where the height is lowered, a distance from an upper side of the core to a case surface is reduced, a heat resistance is lowered, and a heat radiation property is improved.